Many applications exist for battery-powered, digital wireless transmitters, primarily in cellular communications systems such as those operating under the International Telecommunication Union's Wideband Code Division Multiple Access (WCDMA) standard. Such transmitters use one or more amplifiers, such as a digital pre-power amplifier (PPA) and an external power amplifier (PA), to amplify components of the input signal to be transmitted. These components are in-phase and quadrature components in the case of a Cartesian transmitter.
A highly linear amplifier distorts the signal the least and so is most favored from a standpoint of signal quality. Unfortunately, highly linear amplifiers use relatively large amounts of power and numbers of highly accurate and tightly matched components, making them relatively power consumptive, large and expensive. Though they perform the best, they are thus disfavored in many wireless applications, particularly those that require low-cost transmitters or transmitters that are subject to large operating voltage excursions. The amplifier that is best suited overall for low-cost, battery-powered wireless transmitters is a simpler amplifier having significant nonlinearities. See, for example, FIG. 1A, in which a nonlinear amplifier 110 distorts a substantially sinusoidal input signal.
Predistortion is often used to compensate for these nonlinearities, resulting in a linearization of the output of the amplifier. The theory underlying predistortion is that, if an amplifier's distortion characteristics are known in advance, an inverse function can be applied to an input signal to predistort it before it is provided to the amplifier. Though the amplifier then distorts the signal as it amplifies it, the predistortion and the amplifier distortion essentially cancel one another, resulting in an amplified, output signal having substantially reduced distortion. See, for example, FIG. 1B, in which a digital predistorter 120 predistorts the substantially sinusoidal input signal such that the output signal is likewise sinusoidal.
In digital transmitters, digital predistortion (DPD) is most often carried out using a lookup table (LUT) that associates output values with input signal values. Entries in the LUT are addressed using samples of the input signal. The output values retrieved from the LUT are used either to modify the samples (an “inverse gain” configuration) or in lieu of the samples (a “direct mapping” configuration). In modern applications such as WCDMA, samples are transmitted at a very high rate. Thus, the predistorter needs to be able to look up and retrieve output values very quickly.
WCDMA Cartesian transmitters suffer nonlinearities resulting from both amplitude modulation (AM) and phase modulation (PM), namely AM-AM and AM-PM interactions, occurring in their amplifier(s). In such Cartesian transmitters, predistortion is carried out at least partially to negate the effect of these interactions.
Values for a nominal predistortion LUT are typically computed during initial factory calibration. Unfortunately, a factory-calibrated predistortion LUT often fails to linearize the amplifier(s) adequately under varying operational conditions (e.g., temperature, voltage, frequency and voltage standing-wave ratio, or VSWR). Aging, especially in WCDMA and other so-called “3G” transmitters, only exacerbates the inadequacy.